This invention relates to atomic absorption spectroscopy and, in particular, to an improved technique for monitoring and controlling the rate of deposition of a material upon a substrate.
As disclosed in U.S. Pat. Nos. 3,734,620 and 3,654,109, prior art deposition monitor and control systems have been developed which utilize atomic absorption spectroscopy principles. In these systems, selected spectral emission lines absorbable by an evaporant are directed through an evaporant flow. The atomic absorption of the flow material is deduced from the ratio of the transmission of one or more absorbable lines through the evaporant to the transmission of the line or lines through the same vapor free path. Although absorption relates technically to the number density of the evaporant atoms in the optical path of the system, it can be correlated to the rate of deposition of the evaporant upon a specific substrate as explained in the noted patents. Accordingly, if all other system parameters are held constant, each of the transmission measurements can be considered essentially a function of the number density and the correlated rate thus used as a valid indication of the actual deposition rate.
It has been found, however, that not all system parameters remain constant during the deposition process. The alignment of the optical elements, which are typically attached to the vacuum chamber, oftentimes changes during the deposition process due to thermal expansion of the chamber. Misalignment or displacement of the optical elements has been shown to cause changes in observed absorption as high as 15% of the total absorption measurement. Atomic absorption deposition monitoring systems heretofore have been unable to separate changes in transmission due to changes in evaporant density (deposition rate) from those due to misalignment of the optics. Consequently, under certain conditions, the transmission measurements required for precise deposition monitoring and control cannot be acquired without interrupting the process so that new zero absorption readings can be taken. Interrupting a deposition run prior to its completion is generally not practical.